The field of the invention relates to telecommunication systems and more particularly to the encoding of voice signals transmitted over telephone trunk lines.
Long distance telephone calls in the U.S. are typically established over a trunking network of T1 lines. In Europe, the trunking network is made up of E1 lines.
Under either the T1 or E1 format, a trunk channel typically has a capacity of 64 kb/s. Where a subscriber needs a full 64 kb/s, the entire trunk channel may be allocated as a subscriber channel. Where a subscriber requires less than 64 kb/s (e.g., 8 kb/s for a voice channel), the trunk channel may be divided into a number of subscriber channels.
In use, a subscriber channel may be used for either voice or data. While data typically originates in a digital format, voice usually does not. Voice originates in most cases under an analog format from a subscriber""s telephone and is converted into a digital format, either at a local switching office or at a gateway to the trunking network. When the voice signal under a digital format reaches a listener""s telephone, it must be again converted back to an analog format.
The conversion from analog to digital is referred to as compression, while the conversion back to analog is referred to as expansion. The overall compression and expansion of the signal is referred to as companding.
To ensure proper system-wide operation, the overall companding process at each end of the telephone system must be compatible. In the U.S. and Japan, companding occurs under an A-law format. In Europe and most of the rest of the world companding occurs under a xcexc-law format.
In international calls between the U.S. and Europe, companding may occur more than once between speaker and listener because of the differences in companding formats. Further, equipment manufactured in the U.S. and sold in Europe (or visa versa) must be able to recognize and adapt to a local companding format. Because of the importance of telecommunications, a need exists for a simplified method of recognizing a companding format, that is operable from a board level within a communications processing system.
A method and apparatus are described for selecting a companding format of a series of pulse code modulated (PCM) data words encoded under either an A-law or xcexc-law companding format. The method includes the steps of expanding the PCM word into an averaged waveform sample value using both the A-law and xcexc-law companding formats and selecting the companding format providing the expanded average sample value with a smallest relative absolute value.